It has been traditional in the art of sheet-fed printing press machines to provide devices for supporting freshly inked sheet material when transferring the sheet material from one printing station to another or when handling the sheets as they are transferred from the final printing station to a delivery station where the sheets are released and stacked. Typically, a transfer system denotes an apparatus disposed between the several printing stations in the press and which functions to receive a freshly printed sheet from one impression cylinder and move the sheet to the next printing station for additional printing by a further impression cylinder. A delivery system typically denotes an apparatus which receives the freshly printed sheet from the last impression cylinder of the press, and delivers the sheet to the press delivery station, typically a sheet stacker. As used hereinafter, the term transfer is intended to include both apparatus used to transfer a sheet between printing stations of the press and an apparatus used for delivering the sheets to the press delivery stacking station.
In sheet-fed rotary printing presses, it is customary to transfer the sheets from the impression cylinder of one printing station to the impression cylinder of the next by means of one or more successively coacting transfer cylinders, each of which is provided with grippers for engaging the leading edge of the sheet. These cylinders usually are formed with substantially continuous peripheral surfaces for supporting and controlling the body of the sheet during its travel between stations. This transfer apparatus has proven to be effective for transferring sheets in precise registration, but has a tendency to cause the sheets to be marked or smeared.
Marking and smearing of the freshly printed ink occurs as follows. As each sheet is removed from the impression cylinder, and after having received an inked impression, it is immediately conveyed in a reverse curvilinear path with its printed face in contact with the surface of the transfer cylinder. Movement of the sheet is so rapid that the ink on the sheet does not have time to set before it contacts the transfer cylinder surface; consequently, a portion of the ink accumulates on the transfer cylinder surface. As the next sheet and all subsequent sheets are transferred, they may become marked or smeared by the ink accumulation on the cylinder surface.
Marking or smearing of the printed side of the sheet is sometimes caused by fluttering displacement of the sheet as it transfers through the reverse curvilinear path from the impression cylinder to the next transfer cylinder. Slight lateral fluttering in the nip region between the impression cylinder surface and the transfer cylinder surface occurs because of the sudden reversal in the direction of forces acting on the mass of the sheet as it is pulled through the nip region along the reverse curvilinear path. Moreover, the trailing and portion of the wet, printed side of the sheet may be slapped against the transfer cylinder as it is pulled through the nip region. Both the fluttering movement and the tail slap can cause marking or smearing as the freshly imprinted side of the sheet is contacted against the transfer cylinder.